let str = 'border-bottom-color';
function transformStr(str) {
  var strArr = str.split('-');
  for (let i = 1; i < strArr.length; i++) {
    strArr[i] = strArr[i][0].toUpperCase() + strArr[i].slice(1);
  }
  return strArr.join('');
}

console.log(transformStr(str));